The present invention relates to a process for the preparation of 1,3-propanediol (PD) based on hydration of acrolein to 3-hydroxypropionaldehyde (HPA) with subsequent catalytic hydrogenation. According to the present invention, 1,3-propanediol is also obtained from the 3,3'-oxybis-1-propanol (OD) which arises as a by-product of this process.
1,3-propanediol has a wide variety of potential applications as a monomer structural unit for polyesters and polyurethanes and as a starting material in cyclic compound synthesis.
Various processes are known for the preparation of 1,3-propanediol, and these either take as their starting point a molecular structure from a C.sub.2 and C.sub.1 structural unit or, preferably, start directly from a C.sub.3 structural unit (e.g., acrolein). The processes for the preparation of 1,3-propanediol which take acrolein as their starting point are based on two reaction stages: (a) hydration of acrolein in the presence of an acid hydration catalyst and (b) catalytic hydrogenation of the reaction mixture from stage (a) which contains 3-hydroxypropionaldehyde and from which unreacted acrolein has been removed. The reaction mixture of stage (b) contains 3,3'-oxybis-1-propanol (Chem. Abstr. Registry No. 2396-61-4; also known as 4-oxa-1,7-heptanediol or bis(3-hydroxypropyl)ether) in addition to 1,3-propanediol, water and by-products which boil above the boiling point of 1,3-propanediol. The reaction mixture of stage (b) is treated by distillation to yield pure 1,3-propanediol.
The equations for stages (a) and (b) are as follows: ##STR1##
As disclosed by U.S. Pat. No. 2,434,110 (incorporated by reference), hydration can be performed at a raised temperature with use of from 5 to 30 wt % solution of acrolein in water in the presence of an acid (e.g., sulphuric acid, phosphoric acid or acid salts of these acids) with 3-hydroxypropionaldehyde being produced. Hydrogenation of the reaction mixture from which unreacted acrolein has been removed can be performed over conventional hydrogenation catalysts containing one or more metals which can bring about hydrogenation, for example Fe, Co, Ni, Cu, Ag, Mo, W, V, Cr, Rh, Pd, Os, Ir, or Pt. The selectivity of the hydration stage has a decisive influence on the 1,3-propanediol yield. Various catalyst systems have therefore been proposed in order to enable hydration with high selectivity to be carried out in a simple manner on an industrial scale. Examples of hydration catalysts which have been proposed include the following: cation exchange resins having phosphonic acid groups (DE-OS 39 26 136; U.S. Pat. No. 5,015,789, incorporated by reference); chelate-forming ion exchangers such as those having methyleneiminodiacetic acid anchoring groups (DE-OS 40 38 192; U.S. Pat. No. 5,171,898, incorporated by reference); acid-base buffers of organic carboxylic acids or phosphoric acid and salts of these acids which result in a reaction mixture pH of from 2 to 5 (DE Application P 41 38 981.6; U.S. patent application Ser. No. 07/980,955 filed on Nov. 24, 1992, now U.S. Pat. No. 5,284,979, is incorporated by reference)); and inorganic support media having basic activity centers, some of which are occupied by a monovalent acid in a form in which it cannot be detached by water (DE Application P 41 38 982.4; U.S. patent application Ser. No. 07/981,324 filed on Nov. 24, 1992, now U.S. Pat. No. 5,276,201, is incorporated by reference))). A mixture of acrolein and water in a weight ratio of from 1 to 2 to 1 to 20 is normally utilized for the hydration and the reaction is carried out either batchwise or continuously at from 30.degree. to 120.degree. C. at a pressure within the range 1 to 20 bar.
Catalytic hydrogenation of the reaction mixture from the hydration stage from which the acrolein has been removed is generally conducted at a pH within the range 2.5 to 6 and at a temperature within the range 30.degree. to 180.degree. C. Hydrogenation is expediently conducted at from 30.degree. to 80.degree. C. until conversion within the range 50 to 95% is achieved, with further hydrogenation to 100% conversion being conducted at from 100.degree. to 180.degree. C. According to the process of German Patent Application 41 32 663.5 (corresponding to U.S. patent application Ser. No. 07/948,718 filed on Sep. 24, 1992, pending, is incorporated by reference), supported catalysts of titanium dioxide on which platinum is present in a finely-divided form are particularly good hydrogenation catalysts, enabling both a high degree of conversion and also high selectivity to be achieved.
A substantial disadvantage of all the processes disclosed hitherto for the preparation of 1,3-propanediol by acrolein hydration and catalytic hydrogenation of 3-hydroxypropionaldehyde (HPA) lies in the fact that various side-reactions, in particular during the hydration stage, diminish the total 1,3-propanediol yield. 3,3'-oxybis-1-propanol and 4-hydroxy-3-hydroxymethyltetrahydropyrane were the principal products detected in the high boiling point fraction (boiling point above that of 1,3-propanediol) during treatment of the reaction mixture from catalytic hydrogenation.